In the development and operation of axial flow gas turbine engines, it is particularly desirable to be able accurately to monitor the clearances between the tips of rotor blades in the engines and their surrounding casings, because excessive blade tip clearances are detrimental to the efficiency of the engine and result in excessive fuel consumption, whilst blade tip clearances which are too small result in damage to the engine. The problem of reduction of engine efficiency with increasing blade tip clearance is particularly acute when turbine blades are not provided with shrouds at their radially outer ends.
The monitoring of blade tip clearances has become important as a research and development tool in the gas turbine industry, and also shows promise of becoming important in operational engines to give warning of dangerously small clearances during transient operating conditions. Clearance monitoring devices are also required for use as components in active control systems in order to provide a control signal to, e.g. an electro-mechanical system which maintains the clearance to an optimum value under all operating conditions.
There is a requirement for a blade tip clearance monitoring device which enables high-accuracy real-time clearance measurements to be obtained (preferably for each individual blade or portion thereof as its passes the monitoring station at operational speeds) under the severe operational and environmental conditions in and adjacent to gas turbine flow passages. These severe conditions include very fast blade tip speeds, vibration, and the high pressures, velocites and temperatures of the turbine gases. Whilst being able to cope with these demands, the device should preferably be simple, reliable for long-term operation in an engine in normal airline or military service, cheap, and easy to install, calibrate and maintain. When used in active control systems, it should give an output signal which gives a clear indication of the optimum clearance.